1. Field of the Invention
The present invention relates to a magnetic head, head gimbal assembly and magnetic recording and reproducing apparatus, and more particularly to a magnetic head, head gimbal assembly, and magnetic recording and reproducing apparatus in a microwave assisted recording method.
2. Description of the Related Art
As the recording densities of magnetic recording and reproducing apparatuses typified by magnetic disk drives have been increased, there has been a demand to further improve the performance and properties of magnetic heads and magnetic recording media. As magnetic heads, combined-type magnetic heads are widely used, in which a magneto-resistive (MR) effect element, which is a reproducing element that reads out a data signal, and an electromagnetic coil element, which is a recording element that writes a data signal, are laminated. At present, in heads of this type, attempts are being made to downsize these two elements by micromachining and to improve their properties by using new materials.
The recording layer of the magnetic recording medium is an aggregation of magnetic particles. Generally, a single recording bit is formed with a plurality of magnetic particles. To increase the recording density by reducing magnetic fluctuation on a boundary between recording bits, an attempt has been made to downsize magnetic particles. When magnetic particles are downsized, however, heat fluctuation in magnetization in the magnetic particle is increased as its volume is decreased; this is problematic in that thermal stability in magnetization is lowered.
As one solution to this problem, a change from the intra-plane magnetic recording method to the perpendicular magnetic recording method has been studied; in practice, commercialization has been achieved. With media for use in the perpendicular magnetic recording method, even if magnetic particles are downsized, more heat fluctuation can be easily suppressed by, for example, assuring a certain recording layer thickness, when compared with media for use in longitudinal magnetic recording. Thus, the surface recording density can be substantially improved.
To further improve the recording density, however, it is demanded to further downsize magnetic particles included in the recording layer of the magnetic recording medium for use in perpendicular magnetic recording with and to reliably suppress heat fluctuation. A possible approach to meet this demand is to increase the magnetic anisotropy energy KU of the magnetic particle. If the magnetic anisotropy energy KU is increased, however, the coercive force of the recording layer is increased. In practice, the coercive force of a recording layer with heat fluctuation suppressed is, for example, 5 kOe (400 kA/m) or more. By contrast, a magnetic field strength for writing with a magnetic head is substantially determined from the saturation flux density of the soft magnetic material with which magnetic poles in the magnetic head are formed. Therefore, it is very difficult to perform saturation magnetic recording, which is generally assumed to need a write magnetic field with an intensity that is about twice the coercive force.
As a further solution, energy assisted recording is proposed, in which a recording layer with a large coercive force (large magnetic anisotropy energy KU) and the recording of a signal in a recording layer (the reversal of magnetic particle magnetization) is assisted by giving auxiliary energy to the recording layer during recording to lower the effective coercive force. The recording method in which a microwave magnetic field is used as an assisting energy source is referred to as microwave assisted magnetic recording (MAMR).
In microwave assisted magnetic recording, a self oscillating microwave assisted magnetic recording method and an externally oscillated microwave assisted magnetic recording method are known. In the self oscillating microwave assisted magnetic recording method, a microwave magnetic field is generated with a microwave oscillator placed in the vicinity of the recording element of a magnetic head. In the externally oscillated microwave assisted magnetic recording method, a microwave magnetic field is generated by supplying a microwave exciting current from a microwave signal generating circuit, which is provided independently of a thin-film magnetic head, to a microwave magnetic field generating element placed in the vicinity of a recording element. The externally oscillated microwave assisted magnetic recording method needs a microwave transmission path, through which the microwave exciting current is supplied to the microwave magnetic field generating element, to be provided on a suspension or on the microwave magnetic field generating element forming plane of a head slider.
To efficiently supply a microwave exciting current to a microwave generating element (microwave magnetic field generating element), Japanese Unexamined Patent Application Publication No. 2013-206476 proposes a thin-film magnetic head that can generate a certain microwave magnetic field strength with limited electric power by optimizing wires, in a magnetic head slider, that are connected to the microwave magnetic field generating element. This thin-film magnetic head is characterized in that the microwave magnetic field generating element is almost in a short-circuited end state, so a microwave signal can be efficiently supplied to the microwave magnetic field generating element and a microwave magnetic field can be thereby generated.
As described in Japanese Unexamined Patent Application Publication No. 2010-73297, to improve a transmission loss in a microwave signal, a microwave transmission line provided on a suspension is proposed. The characteristic impedance of the microwave transmission line is controlled depending on a positional relationship between shields disposed at the upper and lower portions of the microwave transmission line and conductive columns that mutually connect these shields, so a microwave signal can be efficiently transmitted.